WASHINGTON – Older voters, a critical component of GOP congressional victories for more than a decade, could end up being a major vulnerability for the party in this year's midterm elections, according to strategists in both parties.

Paradoxically, one reason is the new Medicare drug benefit, which was intended to cement their loyalty.

During next week's congressional recess, Democrats are set to begin a major new campaign to highlight what Rep. Nancy Pelosi, D-San Francisco, the Democratic leader, describes as “this disastrous Republican Medicare prescription drug plan.”

Democratic incumbents and challengers plan nearly 100 public forums around the country, armed with briefing books and talking points on a law that, party leaders assert, “was written by and for big drug companies and HMOs, not American families.”

Recognizing the widespread criticism of the new drug program, Republican senators met in a closed session with administration officials last week to discuss the rocky rollout of the plan and prepare for questions back home.

But pollsters say the Republicans' difficulties with the over-60 vote go beyond the complicated drug benefit, which began Jan. 1. President Bush's failed effort to create private accounts in Social Security last year was also unpopular with many older Americans.

That, in addition to confusion over the drug benefit, has “taken the key swing vote that's been trending the Republicans' way and put it at risk for the next election,” said Glen Bolger, a Republican pollster. “And what that means is Republicans are going to have to work extra hard.”

Retirees loom large in midterm elections because they turn out in force at the polls, even in nonpresidential years.

Experts note that the retiree vote is hardly monolithic, nor is it motivated purely by what happens to programs for older Americans. “It's not always economics that prevails,” said Susan MacManus, an expert on generational politics at the University of South Florida in Tampa. She noted that many retirees in her region are younger and more affluent, less dependent on Social Security, Medicare and Medicaid.

But for now, the major battleground is the new Medicare benefit, a program potentially affecting 42 million older and disabled Americans. Surveys show that older voters remain skeptical of the program: A new nationwide poll by the Kaiser Family Foundation, a nonpartisan health research group, found that retirees were almost twice as likely to say they viewed the benefit unfavorably (45 percent) as favorably (23 percent.) Last month's New York Times/CBS News Poll found that most did not expect the law to lower drug costs over the next few years.

At stake is control of the House and Senate: Democrats could gain control of the House for the first time in 12 years if they make a net gain of 15 seats, a difficult challenge. They could regain control of the Senate by picking up six seats.

Older voters will play a crucial role in some marquee races, including the Pennsylvania race between Republican Sen. Rick Santorum and his Democratic challenger, state Treasurer Robert Casey Jr. Among the fewer than three dozen House districts considered competitive, the over-60 vote will be critical in states like Florida and New Mexico.